Imaging tube with sensitivity threshold

ABSTRACT

An electron imaging device which is capable of recognizing, on an instantaneous basis, only those regions of a two-dimensional optical image having a light-intensity exceeding a preselected threshold and which is insensitive to regions of light-intensity less than said threshold.

K 11] 3,745,494 rost 1 1 July 10, 1973 [54] lMAGlNG TUBE WITH SENSITIVITY 2,544,754 3/1951 Townes 315/11 X THRESHOLD 2,928,969 3/1960 Schneeberger 313/65 3,444,413 5/1969 Harrold 315/10 X [75] Inventor: Munsey E- Cro t, ep 3,457,451 7/1969 Manley 315/10 x Assigneez The Unied States oi America as 3,474,286 10/1969 Hcrgenrother 315/11 represented by the Secretary of the Army washmgton Primary Examiner-Carl D. Quarforth [22] Filed: July 23, 1971 Assistant ExaminerP. A. Nelson pp No: 165,718 Attorney- Harry M. Saragovnz. Edward J. Kelly 01111.

Related US. Application Data [63] Continuation-iii-part of Ser. No. 838,283, July 1, .Q. 1969, abandoned. ABSTRACT [52] U.S. Cl 315/11, 315/12, 315/22 An electron imaging device which is capable of recog- [51] Int. Cl. HOlj 31/48 nizing, on an instantaneous basis, only those regions of [58} Field of Search 315/1 1, 10, 31 R, a two-dimensional optical image having a light- 315/12, 22 intensity exceeding a preselected threshold and which is insensitive to regions of light-intensity less than said [56] References Cited threshold.

UNITED STATES PATENTS 2,324,534 7/1943 Pierce 178/72 6 Claims, 4 Drawing Figures PAIENIEB JUL 101915 MIZNZ INVENTOR, MUNSEY E, CR

OST

IMAGING TUBE WITH SENSITIVITY THRESHOLD BACKGROUND OF THE INVENTION This application is a continuation-in-part of Ser. No. 838,283, filed on July 1, 1969, now abandoned.

Previous imaging techniques are unsatisfactory for automatic pattern or character recognition, inasmuch as spurious lightsignals, as well as considerable background illumination, are integrated and displayed along with the desired optical signals. In ordinary storage camera-tubes, where integration over a period of time of the optical scene occurs, this problem can be severe, since regions of even relatively low light-intensity can build up electrical charges on the storage surface until they interfere with the desired signals.

The photoelectron imaging tube of the invention, by its internal operations, automatically dichotomizes the elemental areas in an incoming optical image into those areas in which the light-intensity exceeds a predetermined, but adjustable, threshold, and those areas in which the light-intensity fails to exceed the threshold, producing a single level of output signal, visual or electrical, at all areas in which the threshold is exceeded and completely negating all output signals from all areas in which the threshold is not exceeded. It will be noted that, for television or other viewing tubes, one wants a reasonably linear response to variations in intensity, so that the output picture is a true representation of the input scene. In contrast, with the tube according to the present invention, linearity is purposely sacrificed for ability to discriminate entirely against lower level and spurious signals.

SUMMARY OF THE INVENTION This invention relates to an imaging tube with a preselected light-intensity threshold and which will not produce an output image in areas of the image corresponding to portions of the scene having light-intensity less than the preselected threshold. Moreover, small or spurious incoming signals not exceeding the threshold on an instantaneous time basis are prevented from integrating against time to exceed the threshold but are continuously negated to produce zero output signals. An optical image of the scene to be analyzed is focused upon a photocathode, causing photoelectrons to be emitted from elemental portions of said photocathode with current-densities proportional to the intensities of light at the corresponding areas of the light-image. In addition, the device includes a storage-target, adjacent collecting electrodes, and a flood electron source, with accompanying control electrode, interposed in a noninterfering location between the photoemissive cathode and the collector-target assembly. The storage target has a high resistivity in any direction in the plane of the target but may or may not be conductive through the thickness direction of the target. The flood-source provides a very uniform adjustable electron-current density over the entire target surface. The floodcathode potential is maintained at a value, with respect to a mesh-grid essentially in contact with the storagetarget, such that the flood-electrons arrive at this grid with energies corresponding to a secondary-electronemission ratio slightly less than unity (just below the first crossover). Under these conditions, the exposed areas of the target will charge negatively, essentially to flood-cathode potential. The photoelectron-pattern from the photocathode is accelerated and focused onto the same target. The photocathode potential, relative to the target, is such that photoelectrons arrive at the target with energies corresponding to a secondaryelectron-emission ratio greater than unity, tending to charge the target in a positive direction, because of the induced flow of electrons away from the target. The flood-current density, however, is adjusted to be sufficiently great that, in the case of photoelectrons arriving from areas of the photocathode corresponding to lightintensities below a certain threshold, the net electron current leaving the regions of the target impinged upon by said photoelectrons because of said impingement, which is equal to the product of the photoelectron current to said regions multiplied by (8,, l), where 8, is the secondary-electron-emission ratio of the target at the particular impingement energy of the photoelectrons, is less than the net electron current arriving at these same regions of the target because of impinge ment upon said regions by the flood electrons, which, in turn, is equal to the product of the flood electron current to said regions multiplied by (l 6,), where 8,, is the secondary-electron-emission ratio of the target at the particular impingement energy of the flood electrons; the tendency of the target to charge positively is prevented, and the affected elemental region of the target will recharge to the flood-cathode potential. For those regions of the target, corresponding to areas of the optical image wherein the light-intensity exceeds the preselected threshold-intensity, which receive a photoelectron current of density greater than the predetermined level, depending upon the floodcurrent control-electrode potential, the net electron current leaving said regions of the target because of impingement thereon by said photoelectrons will be greater than the net electron current arriving at said regions of the target because of impingement thereon by the flood electrons; thus the potentials of these regions will be driven rapidly in a positive sense until said potentials exceed the first crossover with respect to the flood cathode. The flood-beam then will strike these target regions at the higher voltage, causing these regions of the target to charge to the collector potential (a potential higher than the first crossover with respect to the flood-cathode potential), as contrasted with the remaining regions of the target, which remain at the much lower flood-cathode potential. In this manner, a charge-pattern is formed on the input side of the target, corresponding to two levels (above and below the preselected threshold) of input information. The selected information on the target then can be read off visually or indirectly, depending upon whether the target includes a phosphor screen or is intended to be scanned by an electron beam, as in a camera-tube, to produce an electrical signal in an output circuit.

If, for example, the target is itself a phosphor screen or contains a layer of phosphor, only those areas that were charged to collector-potential will receive electrons of sufficient energy to cause luminescence when impinged upon by the flood-beam. If only an electrical output is required, with no local display, an electron beam from an electron gun similar to that of an imageorthicon can be made to scan the opposite side of the target. A replica of the voltages produced by the charge-pattern on the input side of the target will appear by capacitive influence on the opposite side, but with considerably smaller amplitude, because of capacitive division of the voltage, resulting from the presence of the target-mesh on this side. The regions of the target of higher potential will cause the target-mesh to collect a substantial portion of the electrons from the beam as the latter scans over such regions. The electrons of the scanning-beam which pass over the lowerpotential regions of the target will nearly all return toward the electron gun substantially along their initial paths and will encounter an electron-multiplier having an output electrode from which an output voltage can be derived.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the electron-tube comprises an envelope 11 within which is disposed a photoemissive cathode 15, upon which an optical image of the scene to be analyzed is formed. The scene to be displayed is represented schematically as an arrow 12. A lens 14 can be used to focus light from the various portions of the scene onto the photocathode 15. It should be understood that the scene to be analyzed may comprise points of many different levels of illumination and that the scene may include spurious light-sources of relatively low light-intensity. The photocathode 15 is maintained at a high negative potential, for example, about minus 1,000 volts, relative to a storage-target 20. The target 20 is highly insulating and has a maximum secondary-emission ratio several times greater than unity. The image focused on the photocathode 15 gives rise to emission of photoelectrons therefrom in currentdensities proportional to the intensities of light in the optical image. This photoelectron-pattem is accelerated and focused, either electrostatically or electromagnetically, upon the storage-target 20 by means now to be described. An electrostatic imaging system in FIG. 1 includes a cylindrical electrode 23 maintained at or near photocathode-potential relative to the target 20, for example, minus 900 volts, and roughly coneshaped anode-electrode 24 held at a positive potential, which, for example, can be about 100 volts positive with respect to the target 20. The electrode 24 contains a small aperture 19 at its apex, which is at or near a crossover along the photoelectron-beam path, outlined by the light solid lines 26 and 27, to allow passage of the photo-electron-beam. Further acceleration of the photoelectronbeam 26, 27 is provided by an accelerator or collector grid 25 positioned relatively close to the target and maintained somewhat more positive than the cone-shaped electrode 24, for instance, at about 150 volts positive relative to the target 20.

Inserted between the accelerator-collector grid 25 and the target 20 is a retarder grid 30 which is maintained at,a voltage just below the first crossover point V, of the secondary-emission ratio vs. voltage curve of FIG. I, for example, at about plus 30 volts. The photoelectrons, which pass through the accelerator-collector grid 25, encounter a retarding field in the region between the electron-transparent accelerator-collector grid 25 and the less positive retarder grid 30. The retarder grid 30 has the purpose of insuring that at the initiation of operation the flood-beam impinges upon the target with energy below the first crossover V The potential of the photocathode 15 is kept sufficiently highly negative relative to the target 20, so that the accelerated photoelectrons from the photocathode l5 impinge upon the target 20 at a relatively high energy level for which the secondary emission ratio 5,, of the storage target for these photoelectrons exceeds unity (above the first crossover and below the second crossover), thereby causing a net average loss of 6,, l electrons from the target for each photoelectron impinging thereon. The areas of the target surface 21 impinged upon by these high energy photoelectrons would charge positively if it were not for the action of the electrons from the flood-cathode 42, to be described subsequently.

The photoelectron current arriving at a given elemental area n of the target is given by the product of J,,,,, the photoelectron current density at the elemental area n, and the area n, or n(J,,,,). Each photoelectron causes emission of 6,, secondary electrons from that area, thus resulting in a secondary-emission current of n(J,,,, 8,) from the area n. Therefore, the net electron current leaving the area n, resulting solely from impingement of photoelectrons upon said area 11, is given by n(J,,,,) (8,, 1). The photoelectron current density J, is itself proportional to the intensity of that portion of the optical image pattern that is focused upon the corresponding area of the photocathode.

A ring-shaped flood electron gun 40 is mounted within the approximately toroidal portion 29 of the cone-shaped accelerating electrode 24 so as not to interfere electron-optically with the electrons from the photo-cathode 15. The flood electron gun 40 includes a flood-cathode 42, a control-grid 44, and an anodegrid 45 for accelerating the electrons from floodcathode 42. The anode-grid 45 may be connected to the accelerating and beam-forming electrode 24. The flood electron beam, outlined by dashed lines 46 and 47, which emanates from the flood-cathode 42, after passage through the control-grid 44 and the anode-grid 45, covers the entire active area of the target 20 with a very uniform electron-current density. The floodcathode 42 can provide a much higher current-density beam than the photocathode 15. The current-density J of the flood electron beam is controlled to lower values than the maximum available by varying the voltage applied to the control-grid 44. The electrons from the flood-cathode 42, like those from photocathode 15, pass through the accelerator-collector grid 25 and retarder grid 30 and impinge upon the surface 21 of the storage-target 20 with an energy level which is low rela tive to that of the photoelectrons. The behavior of the storage-surface 2! of target 20 under electron bombardment depends upon the secondary-electronemission characteristic of the storage-surface material. A typical characteristic curve of secondary-emission ratio vs. voltage, including characteristic curves for various collector voltages such as V and V is shown in FIG. 2. The potential of the flood-cathode 42 relative to the retarder electrode 30 is such that the floodelectrons arrive at the target 20 with an energy level below the first crossover potential V of the curve of FIG. 2; that is to say, the secondary-emission ratio 8, of the storage-surface 21 for these relatively low velocity electrons is less than unity. In other words, an average of fewer than one electron will leave the target for each flood electron impinging thereon.

The flood-current density, assumed to be uniform over all areas of the target, is designated by J,. The flood'current arriving at an area n is given by n(J,). This flood'current causes emission of n(J,) (6,) secondary-emission current. Since 8, is less than unity, the net electron current to the area it caused solely by impingement thereon of the flood electrons is n(J,) (l 8,).

In practice, the current n(J,) (l 6,) is adjusted to be considerably greater than the current n(J,,,,) (6,, l which results from photoemission from areas of'the photocathode corresponding to portions of the optical image of light-intensity below threshold. Then the potential of the elemental area n of the target cannot exceed the first crossover potential but will attain a dynamic equilibrium potential of some value between the first crossover potential relative to the flood-cathode and the potential of the flood-cathode itself. If the photoelectron current were to cease, as it might for a spurious noise signal or a weak input signal that disappeared, then the potential of the elemental area of the target would revert to the lower of the two stable potentials, i.e., the potential of the flood cathode 42. If, on the other hand, the current n(J,,,,) (8,, 1) results from photoemission from areas of the photocathode upon which bright optical signals are imaged, i.e., optical signals above the preadjusted threshold, the current density J for these areas would be considerably greater and of such magnitude that the electron current n(f,,,,) (8,, l) leaving the area n because of impingement thereon by the photoelectrons exceeds the electron current n(J,) (l 8,) collected by these areas because of imingement thereon by the flood beam, and then the potential of the localized area of the target relative to the flood source is driven above the first crossover. The flood electrons then impinge upon the area n of the target with a secondary-emission ratio for the flood electrons greater than unity, thus causing the calized area of the storage target to attain the upper stable potential, that is, the potential of the collector electrode.

With no light impinging upon the photocathode, the storage target operates at the lower of two bistable potentials, essentially that of the flood cathode. When the storage-target has reached the flood-cathode potential, essentially no floodelectron current impinges upon the target.

If an optical image now is projected upon the photoeathode, photoelectrons will be caused to impinge upon the various elemental regions of the target with current densities at said regions proportional to the light intensities imaged upon the corresponding areas of the photocathode. Since the photoelectrons have high energy upon reaching the target, even when the target potential has reached its most negative level, more electrons will leave the target than arrive from the photocathode (considering the effects of the photoelectrons only), in the ratio of 6,,:[ (where 8, is the instantaneous secondary-emission ratio, with reference to the photoelectrons) tending to raise the localized target potential above the lower stable potential. Immediately, some of the flood electrons impinging upon these localized areas tend to lower the target potential again. Some secondary electrons will escape from the target, so that the effective number of electrons retained by the target for each impinging flood electron is l 8,, where 8, is the instantaneous secondaryemission ratio of the target, with reference to the floodelectrons.

If the maximum value of the flood current density multiplied by (l 8,) is greater than the photoelectron current-density to a particular elemental area multiplied by 8,, l), the potential of the elemental area cannot rise above the first crossover potential, and, if the photoelectron current ceases, the elemental area again will be recharged to the flood cathode potential (lower stable potential). The same effect would occur with randomly emitted photoelectrons where electron current is of too short a duration to charge the capacitance of the elemental area to a potential above the first crossover potential. This is the principle which prevents integration of false signals against time, which would otherwise result in spurious outputs.

If, on the other hand, the current-density J,,,, in the photoelectron-image at any area n is so large that, when multiplied by (8,, 1), it substantially exceeds the flood current-density J multiplied by l 8,), then the potential of the elemental area n can be driven more positive than the first crossover potential with reference to the flood source. In this case, the floodelectrons now would approach the target and impinge thereon with a 8, greater than unity, thus rapidly driving the elemental area n more positive until it reached the upper stable potential, i.e., the collector potential.

This tube, therefore, operates by means of a bistable equilibrium, and since the flood-current density is specified as adjustable, this current-density can be set to correspond to any suitable value of the light-intensity level of the threshold that will establish the required dichotomy of light levels in the image.

If the surface 21 of the storage-target 20 facing the electron beams is coated with a phosphor or is itself a phosphor, then only those regions of the surface 21 which have been charged to accelerator-collector-grid potential (that is, the regions corresponding to areas of the optical image of greater than threshold lightintensity) will receive electrons with sufficient energy to become luminescent when exposed to the floodbeam.

In FIG. 3, the voltage and current-density relationships against time for a typical false light stimulus, or any signal of intensity below a predetermined threshold, and for a typical true light stimulus, or any signal of intensity above said threshold, are shown. The current-density J of the photoelectron-current multiplied by the quantity (8,, l) for a false signal 50 lies below the flood-current density charging level J which is equal to J,(1 8,), and which is indicated by the dashed line 52, whereas a true signal 51 produces a currentdensity of amplitude greater than this level J The charging current-density level J 1 can be shifted upward by making the potential of the control-grid 44 less negative and can be lowered by making the control-grid potential more negative. The potential-variation V versus time T of the target 20 for the false signal 50 is indicated by the curve 54; it is evident that the voltage has not reached the first crossover potential V at which the secondary-emission ratio of the target is unity, before the stimulus disappears. This first crossover potential, indicated by the solid line 53, is essentially fixed for any given target-surface material. The voltage 54 rapidly returns to the voltage V of the flood-cathode, after recharging of the area by the flood-beam. The voltage at the target for a true signal is indicated by the curve 55, which climbs rather rapidly above the crossover potential V and approaches the voltage V, of the accelerator-collector grid 25. For a true signal 51, therefore, the voltage at the storage-target will attain a steady level substantially equal to that of the accelerator-collector grid 25.

In order to erase the stored information from the target 20, it is necessary only to lower the potential of the accelerator-collector grid momentarily below the first crossover potential V,,, with the flood-current on. The storage-target then will entirely become charged below first crossover potential, and the flood-beam will recharge the surface to flood-cathode potential. The accelerator'collector grid may then be restored to its normal voltage without altering the state of charge.

The phosphor target of FIG. 1 can be viewed directly by an observer or by a conventional camera-tube for electronic transmission elsewhere.

An example of an imaging tube with an electrical output is shown in FIG. 4, wherein portions of the tube corresponding to those of the tube of FIG. 1 (the portions separate from the scanning-section 60) will be designated by like reference numerals with the letter A affixed thereto. In the device of FIG. 4, the operation of the portion of the tube including the target 20A and the elements to the left of the target is the same as for the device of FIG. 1, except that, for reasons which will be evident subsequently, a target-mesh 80 is provided either in contact with or closely adjacent the back surface 22A of the target 20A opposite the photocathode and flood-cathode. In practice, the insulating target 20 can be supported directly by the mesh 80, which is made of metal. The potential of the flood-cathode 42A normally is maintained at approximately the same voltage as the cathode 61 of the scanning-section 60. Just as in the device of FIG. 1, the elemental regions of the storage surface 21A of target 20A attain either the accelerator-collector grid potential or the floodcathode potential, depending upon whether or not the image-intensity is greater than or less than the threshold. The threshold-level is governed by proper choice of the potential of the control-grid 44A, which controls the flood-beam current-density.

The target-mesh 80 is maintained at a potential slightly negative with respect to the scanning-section cathode 61, for example, at about minus 2 volts. In other words, the potential of the target-mesh 80 is made sufficiently negative to insure that electrons from the primary beam of the scanning-section emanating from electron gun 62 are brought to the immediate vicinity of the back of the storage-surface 22A of the target and reach the target-mesh with about zero velocity. These electrons are repelled in the case of regions of the target corresponding to dark areas of the optical image. The bright and dark regions of the front target storage-surface 21A may, for example, be about plus 150 volts and zero volts, respectively. Because of the capacitive effect in the insulating target 20A, some of the voltage on the front storage-surface 211A will be induced through the target 20A to the opposite or back storage-surface 22A. The regions of the back targetsurface 22A corresponding to bright" and dark areas of the optical image can reach potentials of about plus 50 volts and zero volts, respectively. The beamscanning may be effected by any one of several well known electromagnetic or electrostatic deflection means. The scanning-section may be similar to that used in an image-orthicon tube, such as described in an article by Albert Rose et al. in ProcIRE, July, 1946, pages 424-432, entitled The Image Orthicon-A Sensitive Television Pickup Tube, if electromagnetic deflection and focusing are used. If electrostatic deflection and focusing are used, which will minimize interaction between the fields in the scanning-section and the image-section, a design of the scanning-section similar to that described by Kurt Schlesinger and Bernard Day in Final Report, Research and Development of Miniature Environmentalized Electrostatic Image Orthicon,

Contract DA36-O39-AMC-02269(E), General Electric Co., may be applied. The scanning-section 60 includes, in addition to the electron gun 62, suitable beamdeflecting means 64, accelerating means 65, and a retum-beam electron-multiplier 66 having input dynode 67 and output-anode 68. The primary beam from cathode 61 passes through the axial bore in the electronmultiplier, the deflection-means 64, and the positive accelerator 65. The accelerator 65 terminates in an electron-transparent fieldmesh 69 and may, for exam ple, be at about 500 volts. The primary beam from the electron gun 62 of scanning-section 60, upon reaching the field-mesh 69, encounters a decelerating field between the field-mesh and the target-mesh 80. The primary electrons are there decelerated to nearly zero velocity. The deflection means 64, shown schematically in FIG. 4, can include any suitable arrangement of horizontal and vertical deflection-plates connected to a television-type sweep circuit capable of causing the primary beam of electrons from the reading-gun cathode 61 to scan the charge-image on the storage-surface 22A of target 20.

In the scanning-section 60, some of the primary electrons arriving in the vicinity of the target 20A from the cathode 61 will be collected by target-mesh 80, and the remainder will be reflected. The number collected is modulated by the voltage-pattem on the uncovered surface 22A of target 20A. In the regions of the target which are at or near flood-cathode potential on storage-surface 21A (approximately zero volts), substantially all of the electrons from the cathode 61 are reflected and, under the influence of the accelerating field of the accelerator 65, return towards the electron gun 62 generally along their initial paths. Some of these reflected electrons, however, are collected by the positive field-mesh 69 and are removed from the scanningsection 60. The remaining reflected electrons, enroute to the electron-gun end of the scanning-section 60, impinge upon the input-dynode 67 of electron-multiplier 66 at a potential sufficient to generate an increased number of secondary electrons. The multiplier 66, which includes several stages, is indicated schematically in FIG. 4, and the number of electrons available at the output anode 68 of the multiplier is increased by several orders of magnitude. The signal-current passing through output-resistor 70 is relatively high. Resistor 70 may be relatively low in resistance to maintain good high-frequency response, at the expense of signal amplitude. Amplifier 72 is used to increase the amplitude of the signal at output-terminal 75. The number of electrons actually returning to the electron-multiplier, for

a given target-mesh potential, will depend upon the relative potential of the storage-surface 22A and the target-mesh 80. For example, as an area of the storagesurface 22A becomes more positive, more of the electrons from the primary beam approaching that area of the target 20A will be collected by the target-mesh 80, and the return electron-flow reaching the electronmultiplier 66 will be correspondingly reduced. In other words, a division of the return electron-flow between the target-mesh and the electron-multiplier 66 can be achieved by varying the storage-surface voltage. When the scanning-beam passes a region of the surface 22A of target 20A which is at the lower potential, that is, a region of the charge-image corresponding to an area of no signal in the optical image on the photocathode A, the scanning-beam electrons are repelled by this region of the target, and most of the electrons can return to the electron-multiplier 66. This situation corresponds to zero output-signal at terminal 75, even though the current is at its highest level.

When the scanning-beam passes a region of the surface 22A of target A which is at the higher positive potential, that is, a region of the charge-image corresponding to a bright area of the optical image on the photocathode 15A, some of the scanning-beam electrons are allowed to approach and impinge upon the target-mesh 80, and relatively few of the electrons can return to the electron-multiplier 66. During scanning of bright spots of the charge-image on the target, therefore, the current-output at the multiplier 66 decreases markedly, causing a more positive voltage to appear at output-electrode 68. In this manner the voltage at the output-terminal 75 of amplifier 72 provides an electronic reproduction of the voltage-image on the surface 22A of target 20A; consequently the portions of the scene which have an intensity above the threshold are clearly recognized.

What is claimed is:

1. An electron-imaging tube whose output represents only those portions of an optical image of a scene having a light intensity exceeding a preselected threshold and a time duration exceeding a predetermined minimum comprising a photoemissive source emitting photoelectrons in a photoelectron pattern from elemental areas thereof of current density proportional to the intensity of light imaged upon said areas, a storage target of high surface resistivity and a maximum secondaryemission ratio greater than unity, a collector electrode connected adjacent said storage target for collecting electrons emitted by said storage target, means for directing said photoelectrons upon said storage target, said means for directing including means for focusing said photoelectrons from the elemental areas of said photocathode onto corresponding regions of said storage target, said means for directing further including means for accelerating said photoelectrons so as to impinge upon said storage target at an energy level at which the secondary-emission ratio of said storage target exceeds unity; a flood electron source operating simultaneously with said photoemissive source for flooding uniformly the entire active portion of said target with a flood electron beam at a substantially uniform electron energy at which the secondary-emission ratio of said target is less than unity.

2. An electron-imaging tube according to claim 1 further including means including a control grid for adjusting the electron current density of said flood beam to a predetermined level dependent upon said threshold desired, said regions of said target attaining one of two stable operating potentials depending upon whether or not said preselected threshold is exceeded.

3. An electron-imaging tube according to claim 1 wherein said flood source is mounted to avoid interfering electron-optically with said photo-emissive source.

4. An electron-imaging tube according to claim 1 further including means for directing a scanning electronbeam to the immediate vicinity of the side of said target opposite to that upon which the photoelectrons and flood electrons impinge, the major portion of said scanning electron-beam being reflected from regions of said target corresponding to portions of said image of lightintensity less than said preselected threshold, the major portion of said scanning electron-beam being attracted to the vicinity of said target at regions thereof corresponding to said portions of said image of lightintensity exceeding said preselected threshold, and output means responsive to said reflected portion of said scanning electron-beam for deriving a distinctive output voltage when said light-intensity exceeds said preselected threshold.

5. An electron-imaging tube whose output represents only those portions of an optical image of a scene having a light intensity exceeding a preselected threshold and a time duration exceeding a predetermined minimum comprising a photoemissive cathode emitting photoelectrons from elemental areas thereof with current density proportional to the intensity of light impinging upon said areas,

a storage target with high surface resistivity,

means for accelerating the photoelectrons so that they impinge upon the storage target at an energy level for which the secondary-emission ratio 8,, of the storage target for these photoelectrons exceeds unity causing 8,, l electron to emanate from a region of the target for each photoelectron impinging thereon, means for focusing the photoelectrons upon said target so that photoelectrons from a given elemental area n' of said photoemissive cathode impinge upon a corresponding area n of said target and with a current density J which is a function of the intensity of the image at said elemental area n of said photoemissive cathode,

a flood gun for flooding uniformly the storage target with electrons of relatively low energy such that the secondary emission ratio 8, of said target for these flood electrons is below unity causing a net average of l 8, electrons to leave said target for each flood electron impinging thereon, means for controlling the current density J, from the flood gun so that the product of the flood beam current density J, and (l 8,) is less than the product of the photoelectron current density J, and (8, l) at those regions of the target corresponding to areas of the photocathode having an intensity above said predetermined threshold image intensity and vice versa for those regions of the target corresponding to areas of the image on the photocathode having an intensity less than said predetermined threshold image intensity, and a collector electrode adjacent said target for collecting electrons emitted thereby and operating at a potential relative to said target at which the secondary-emission ratio of said target for flood-beam electrons is substantially greater than unity, said storage target having a lower stable potential substantially equal to that of the flood source and an upper stable potential substantially equal to that of said collector electrode, and means including a control grid for adjusting the electron current density of said flood beam to a predetermined level dependent upon said threshold desired, said regions of said target operating at one of two stable potentials depending upon whether or not said preselected threshold is exceeded.

6. An electron-imaging tube according to claim 5 further including means for directing a scanning electronbeam to the immediate vicinity of the side of said target opposite to that upon which the photoelectrons and selected threshold. 

1. An electron-imaging tube whose output represents only those portions of an optical image of a scene having a light intensity exceeding a preselected threshold and a time duration exceeding a predetermined minimum comprising a photoemissive source emitting photoelectrons in a photoelectron pattern from elemental areas thereof of current density proportional to the intensity of light imaged upon said areas, a storage target of high surface resistivity and a maximum secondary-emission ratio greater than unity, a collector electrode connected adjacent said storage target for collecting electrons emitted by said storage target, means for directing said photoelectrons upon said storage target, said means for directing including means for focusing said photoelectrons from the elemental areas of said photocathode onto corresponding regions of said storage target, said means for directing further including means for accelerating said photoelectrons so as to impinge upon said storage target at an energy level at which the secondary-emission ratio of said storage target exceeds unity; a flood electron source operating simultaneously with said photoemissive source for flooding uniformly the entire active portion of said target with a flood electron beam at a substantially uniform electron energy at which the secondary-emission ratio of said target is less than unity.
 2. An electron-imaging tube according to claim 1 further including means including a control grid for adjusting the electron current density of said flood beam to a predetermined level dependent upon said threshold desired, said regions of said target attaining one of two stable operating potentials depending upon whether or not said preselected threshold is exceeded.
 3. An electron-imaging tube according to claim 1 wherein said flood source is mounted to avoid interfering electron-optically with said photo-emissive source.
 4. An electron-imaging tube according to claim 1 further including means for directing a scanning electron-beam to the immediate vicinity of the side of said target opposite to that upon which the photoelectrons and flood electrons impinge, the major portion of said scanning electron-beam being reflected from regions of said target corresponding to portions of said image of light-intensity less than said preselected threshold, the major portion of said scanning electron-beam being attracted to the vicinity of said target at regions thereof corresponding to said portions of said image of light-intensity exceeding said preselected threshold, and output means responsive to said reflected portion of said scanning electron-beam for deriving a distinctive output voltage when said light-intensity exceeds said preselected threshold.
 5. An electron-imaging tube whose output represents only those portions of an optical image of a scene having a light intensity exceeding a preselected threshold and a time duration exceeding a predetermined minimum comprising a photoemissive cathode emitting photoelectrons from elemental areas thereof with current density proportional to the intensity of light impinging upon said areas, a storage target with high surface resistivity, means for accelerating the photoelectrons so that they impinge upon the storage target at an energy level for which the secondary-emission ratio delta p of the storage target for these photoelectrons exceeds unity causing delta p - 1 electron to emanate from a region of the target for each photoelectron impinging thereon, means for focusing the photoelectrons upon said target so that photoelectrons from a given elemental area n'' of said photoemissive cathode impinge upon a corresponding area n of said target and with a current density Jpn which is a function of the intensity of the image at saId elemental area n'' of said photoemissive cathode, a flood gun for flooding uniformly the storage target with electrons of relatively low energy such that the secondary emission ratio delta f of said target for these flood electrons is below unity causing a net average of 1 - delta f electrons to leave said target for each flood electron impinging thereon, means for controlling the current density Jf from the flood gun so that the product of the flood beam current density Jf and (1 - delta f) is less than the product of the photoelectron current density Jp and ( delta p - 1) at those regions of the target corresponding to areas of the photocathode having an intensity above said predetermined threshold image intensity and vice versa for those regions of the target corresponding to areas of the image on the photocathode having an intensity less than said predetermined threshold image intensity, and a collector electrode adjacent said target for collecting electrons emitted thereby and operating at a potential relative to said target at which the secondary-emission ratio of said target for flood-beam electrons is substantially greater than unity, said storage target having a lower stable potential substantially equal to that of the flood source and an upper stable potential substantially equal to that of said collector electrode, and means including a control grid for adjusting the electron current density of said flood beam to a predetermined level dependent upon said threshold desired, said regions of said target operating at one of two stable potentials depending upon whether or not said preselected threshold is exceeded.
 6. An electron-imaging tube according to claim 5 further including means for directing a scanning electron-beam to the immediate vicinity of the side of said target opposite to that upon which the photoelectrons and flood electrons impinge, the major portion of said scanning electron-beam being reflected from regions of said target corresponding to portions of said image of light intensity less than said preselected threshold, the major portion of said scanning electron-beam being attracted to the vicinity of said target at regions thereof corresponding to said portions of said image of light-intensity exceeding said preselected threshold, and output means responsive to said reflected portion of said scanning electron-beam for deriving a distinctive output voltage when said light-intensity exceeds said preselected threshold. 